Genetic Evidence for an Indispensable Role of Somatic Embryogenesis Receptor Kinases in Brassinosteroid Signaling

Abstract

The Arabidopsis thaliana Somatic Embryogenesis Receptor Kinases (SERKs) consist of five members, SERK1 to SERK5, of the leucine-rich repeat receptor-like kinase subfamily II (LRR-RLK II). SERK3 was named BRI1-Associated Receptor Kinase 1 (BAK1) due to its direct interaction with the brassinosteroid (BR) receptor BRI1 in vivo, while SERK4 has also been designated as BAK1-Like 1 (BKK1) for its functionally redundant role with BAK1. Here we provide genetic and biochemical evidence to demonstrate that SERKs are absolutely required for early steps in BR signaling. Overexpression of four of the five SERKs—SERK1, SERK2, SERK3/BAK1, and SERK4/BKK1—suppressed the phenotypes of an intermediate BRI1 mutant, bri1-5. Overexpression of the kinase-dead versions of these four genes in the bri1-5 background, on the other hand, resulted in typical dominant negative phenotypes, resembling those of null BRI1 mutants. We isolated and generated single, double, triple, and quadruple mutants and analyzed their phenotypes in detail. While the quadruple mutant is embryo-lethal, the serk1 bak1 bkk1 triple null mutant exhibits an extreme de-etiolated phenotype similar to a null bri1 mutant. While overexpression of BRI1 can drastically increase hypocotyl growth of wild-type plants, overexpression of BRI1 does not alter hypocotyl growth of the serk1 bak1 bkk1 triple mutant. Biochemical analysis indicated that the phosphorylation level of BRI1 in serk1 bak1 bkk1 is incapable of sensing exogenously applied BR. As a result, the unphosphorylated level of BES1 has lost its sensitivity to the BR treatment in the triple mutant, indicating that the BR signaling pathway has been completely abolished in the triple mutant. These data clearly demonstrate that SERKs are essential to the early events of BR signaling.

RGF1 INSENSITIVE 1 to 5, a group of LRR receptor-like kinases, are essential for the perception of root meristem growth factor 1 in Arabidopsis thaliana

“…The notion that BAK1 and its paralogs may be the co-receptors of RGIs is supported by a few observations. First, serk1 bak1 bkk1 or serk1 serk2 bak1 triple mutants show short root phenotype which is independent of the BR signaling pathway [33,34]. Second, members of RGIs were found to interact with BAK1 in a yeast two-hybrid analysis (data not shown), which was the primary motivation prompted us to study the functions of the entire clade of RGIs.…”

“…Our laboratory is interested in revealing the biological functions of these undefined LRR-RLKs. Our previous genetic analyses indicated that BAK1 and its paralogs are involved in regulating brassinosteroid (BR)-independent root growth and development, as serk1 serk2 bak1 and serk1 bak1 bkk1 triple null mutants show intensively shorter roots compared with BR-deficient or BR signaling-impaired mutants [33,34]. We proposed that BAK1 and its paralogs may serve as a co-receptor and pair with another LRR-RLK to regulate root growth and development, similar to its role in BRI1-or FLS2-mediated signaling pathway [35][36][37][38][39][40][41][42][43].…”

A Mathematical Model for the Coreceptors SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE3 in BRASSINOSTEROID INSENSITIVE1-Mediated Signaling

“…The identification of members of the SERK family that act redundantly in different signaling pathways Li, 2010;Du et al, 2012;Gou et al, 2012;Kim et al, 2013) has underscored the importance Figure 5. Prediction of BRI1 signaling output in the absence of SERK coreceptors.…”

Visualization of BRI1 and BAK1(SERK3) Membrane Receptor Heterooligomers during Brassinosteroid Signaling

“…This inhibitory mechanism involves the BRI1 KINASE INHIBITOR1 (BKI1), which binds to the kinase domain of unliganded BRI1 receptors and thereby keeps the LRR-RLK inactive (Jaillais et al, 2011b). In vitro, BKI1 precludes BRI1 from associating with BRI1-ASSOCIATED KINASE1 (BAK1; also known as SOMATIC EMBRYO-GENESIS RECEPTOR-LIKE KINASE3 [SERK3]; Wang and Chory, 2006;Jaillais et al, 2011b), another LRR-RLK required for BR signal transduction Nam and Li, 2002;Gou et al, 2012). Binding of BRs to the extracellular LRR domain of BRI1 homodimers is thought to result in conformational changes, which trigger basal BRI1 kinase activity and autophosphorylation Kim and Wang, 2010;Jaillais et al, 2011a).…”

“…The first is the observation by Gou et al (2012) that BAK1(SERK3), SERK1, and SERK4 (also known as BAK1-LIKE KINASE1) not only enhance BRI1 kinase activity but are essential for initiating downstream signal transduction. The second derives from the resolved crystal structure of the BRI1 ectodomain and opposes the hypothesis that BRI1 homodimers function as ligandperceiving receptor complexes.…”